This invention relates generally to apparatus and methods used for the detection of topographical variations on a surface of a data storage medium using the thermal response of a sensor moving relative thereto. More particularly, the invention relates to apparatus and methods for enhancing the time and spatial resolution of-the sensor used to detect the variations to more accurately locate and characterize variations on or in the surface.
In data processing systems, magnetic disk drives and other media are often used as direct access storage devices. In such devices, read-write heads are used to write data on or read data from an adjacently rotating hard, flexible disk, tape, or other form of data storage media. In the following discussion, the term xe2x80x9cdiskxe2x80x9d will be used almost exclusively, but it is to be understood that the discussion is not limited to disk-like media but all similar data-storage media are included.
Read-write heads typically travel over the surface of the disk at relatively high speeds and in close proximity to the surface of the disk. To prevent damage to either the disk or the read-write head, it has been recognized for a long time that the surface of the disk should be very flat and free of any bumps, asperities, or the like which might be contacted by the read-write head. Also, the read-write heads are typically designed so that they fly over the surface of the rotating disk at a very small, though theoretically constant distance above the disk, the separation between the read-write head and the disk being maintained by a film of air. During its flight, the head undergoes continuous vibration, pitch, and roll as the topography of the disk varies beneath the head. If the quality of the disk or the read-write head is poor, occasional rubbing or sharp contact may occur between the disk and the read-write head, leading to damage to the head or to the disk, or loss of valuable data, or all of these which are typically referred to as xe2x80x9chead crashesxe2x80x9d.
Various attempts have been made to provide increased assurance that such undesirable contact between a read-write head and a recording disk does not occur. Rigid manufacturing and quality assurance specifications for both the recording disk and the read-write head have been developed to minimize or prevent such contact.
Disk inspection for various types of defects, including magnetic, optical and topographic defects (i.e., delamination, voids, inclusions, asperities, etc.), is of critical importance for the increasingly stringent production requirements facing a manufacturer today as smaller drives store more data and operate at higher speeds. Many methods of inspection to find defects are in use, and many more have been proposed. These include optical techniques (fiber interferometry, bulk optic shear interferometry, microISA), magnetic readout (simply screening, HRF, etc.,) and mechanical testing (for example, the PZT glide test). Each of these techniques may play a role in achieving the goal of the virtually defect-free production of magnetic disks. However, with a tightening market and more exacting technical requirements as heads fly lower and faster, less expensive and more accurate inspection schemes become more significant.
U.S. Pat. No. 4,747,698 to Wickramasinghe, et al. is directed to an inspection apparatus referred to as a Scanning Thermal Profiler. In the method using this apparatus, a fine scanning tip is heated to a steady state temperature at a location remote from the structure, for example, the surface, to be investigated. Thereupon, the scanning tip is moved to a position proximate to, but spaced from the structure. At the proximate position, the temperature variation from the steady state temperature is detected. The scanning tip is scanned across the surface structure with the aforesaid temperature variation maintained constant. Piezo-electric drivers move the scanning tip both transversely of, and parallel to, the surface structure. Feedback control assures the proper transverse positioning of the scanning tip and voltages thereby generated replicate the surface structure to be investigated. While this approach provides excellent height resolution, it requires the use of an expensive scanning tip. This technique also has the disadvantage that it cannot readily be utilized on an assembled disk drive.
Thermal Proximity Imaging (TPI) or Sensing (TPS) is a surface inspection technique by which topographical variations on the surface of a medium can be detected and characterized by monitoring the thermal response of a sensor in motion relative to the surface. Topographical variations may be asperities, projections, recesses, voids, particles, impurities, etc., or any other geometric or material deviation from a desired relatively smooth surface of relatively uniform composition which can produce a measurable change in the temperature of an object passing over the surface. As disclosed in the above-identified U.S. Pat. No. 5,527,110, a sensor, possibly a magnetoresistive (or MR) access element used to access data on a data storage medium, is heated using a known electrical current, or xe2x80x9cbias currentxe2x80x9d. The Joule-effect heat induced in the sensor by the bias current varies when topographical variations on the surface pass by the sensor during the relative movement between the sensor and the surface. Because the topographical variations vary the distance (that is, xe2x80x9cheightxe2x80x9d or xe2x80x9cgapxe2x80x9d) between the sensor and the surface, and because the heat transferred from the sensor varies as a function of this distance, measuring or monitoring the temperature change of the sensor is a useful technique for identifying the location and character of topographical variations on the medium. TPS is a very sensitive and reliable indicator of disk surface topography. TPS can serve as an imaging tool, defect detector, and a sensitive indicator of head dynamics. TPS has been shown to be a useful tool for xe2x80x9cin situxe2x80x9d measurement of disk topography and for defect detection. It is a non-invasive technique. The sensitivity of the TPS method is in the sub-nanometer range for height, and the thermal response is in the MHz range for presently-existing head geometries and bias current arrangements.
However, existing TPS methods typically are inherently limited by the relatively large thermal signatures produced during Joule heating of the sensor. The area on the surface being examined which is heated during the heating of the sensor, for example, by a direct current power source, is typically much larger than the sensor itself. For example, a sensor having dimensions of about 0.1 micronsxc3x973 microns will typically heat an area of about 3 micronsxc3x976 microns on the surface being examined. This broad thermal footprint, or heat-effected zone, relative to the sensor, detracts from the accuracy with which topographical variations can be located. For example, a topographical variation effecting a change in the sensor temperature may be 1.5 microns or more away from the location of the sensor. Furthermore, the detection and locating of smaller variations within the thermal footprint may be obscured by the presence of larger variations that also lie within the footprint.
Thus, in using conventional TPS devices, the detection of small topographical variations and the locating of all topological variations can be hampered due to the broad thermal footprints typically produced in such systems. As a result, the sensitivity or resolution of the detection method suffers. Therefore, there is a need in the TPS art to provide methods and devices for limiting the dimensions of the area (or volume) heated by the Joule-heating of the sensor in order to improve the sensitivity of the TPS method.
In addition, in order to provide quantitatively accurate images of topographical variations on a surface over which a TPS sensor is to be moved, it is also desirable to provide TPS techniques which support sensors designed for non-invasive, in-situ measurement of topography in storage systems, and should therefore, preferably, not require any particular, non-standard use or manipulation of the surface itself.
The desired thermal proximity sensor techniques and devices are provided by the instant invention. One embodiment of this invention is, for example, a TPS technique (method and system) wherein the resolution of the detection of topographical variations on surfaces is improved by varying the heating of the sensor object. Limiting the energy provided to the sensor object, for example, by supplying the energy in the form of energy pulses of limited duration, that is, limited xe2x80x9con-timexe2x80x9d, limits the diffusion of heat to areas of the surface under investigation to the immediate vicinity of the sensor object. Thus, asperities located beyond the immediate vicinity of the sensor are less likely to influence the temperature of the sensor. This limiting of the thermal footprint of the TPS technique improves the sensitivity or resolution of the technique.
In addition to limiting the duration of the energy pulses provided to the sensor, in another embodiment of the invention, the time at which no energy is provided to the sensor object, that is, the time that no heating of the sensor object occurs, is optimized. By optimizing the time at which no heating occurs, that is, the xe2x80x9coff-timexe2x80x9d, the temperature of the adjacent medium under investigation is allowed to decrease and limit the area (or volume) to which heat diffuses in the subsequent energy pulse. This, again, ensures that only topological variations in the immediate vicinity of the sensor object influence the temperature of the sensor object. As a result, the sensitivity of the detection method and the accuracy with which variations can be located are enhanced.
Another embodiment of the invention is, for example, a method for mapping the location of surface variations on a substantially planar surface comprising: moving a sensor object over the planar surface at a substantially constant height above the surface; supplying energy to said object to thereby raise the temperature of the object and heat at least part of the planar surface; varying the supply of energy to said object to thereby limit the size of the area heated on the planar surface; and detecting a change in temperature of said object when the object is in proximity to a surface variation to define the location of said surface variation. The step of varying the supply of energy is typical practiced by means of providing energy pulses to the sensor object. The pulses may typically have an xe2x80x9con-timexe2x80x9d and an xe2x80x9coff timexe2x80x9d.
A further embodiment of the invention is, for example, an apparatus for mapping the location of surface variations on a substantially planar surface having a measurement resolution, the apparatus comprising: a slider configured to slide at a substantially constant height over the planar surface; a sensor object carried by said slider in close proximity to said planar surface; an energy supply configured to supply energy to the said object to thereby raise its temperature, wherein said energy is supplied as energy pulses, and wherein said energy pulses are configured to generate heat pulses within said planar surface wherein the thermal diffusion dimension of said heat pulses in said planar surface is less than the measurement resolution; a detector configured to detect a change in temperature of said object when it is in proximity to a surface variation; and means for defining the location of the surface variation from the detected change in temperature of the object.
Still another embodiment of this invention is a method for mapping the location of surface variations of relatively small height on a substantially planar surface comprising the steps of: supplying energy to an object in close proximity to the planar surface to thereby raise the temperature of the object; moving the object with respect to the planar surface while keeping the object a substantially constant distance from the planar surface; and detecting a change in temperature of the object when the object is in proximity to a surface variation to define the location of the variation; wherein pulses of energy are supplied to the object to improve the resolution with which the location of the variation can be defined.
The on-time and off-time of the energy pulses may be constant or vary in duration. The on-time may be constant and the off-time vary, or the on-time may vary and the off-time be constant. In addition, the timing of the pulses may vary with respect to the relative position of the sensor object on the planar surface under examination. For example, the timing of the pulses my vary with respect to a fixed position on the planar surface.
The variable heating of the sensor is preferably practiced by means of varying electric current, though other forms of heating, for example, by laser, can also be used.
The methods and apparatus of the present invention provide means for detecting, locating, and characterizing topographical variations with improved spatial resolution compared to the prior art.